More Details on the New Mastery System
As mentioned previously, a new World of Warcraft system called Mastery is going to be implemented in Cataclysm, along with all of these attribute adjustments. Mastery does, in fact, make you better at whatever it is you do, whether its DPS, Healing, or Tanking, and even gets into specific bonuses depending on your talent tree focus.
Mastery is earned purely by investing points into talent trees. The goal of this is to make it so that talents are more fun and interesting, and much less "bonus" based. For example, Fire spec'd Mages currently pick up a talent called Fire Power that does nothing more than increase their fire damage by 10%. Another example is the Death Knight talent Guile of Gorefiend, which has two completely unrelated bonuses - one as increased critical strike damage and the other as increased duration on a mitigation ability.
With Mastery, talents that do nothing more than offer "required" bonuses, as well as talents that are multitasking, will be modified or removed. Instead, simply by investing points into a particular talent tree you will earn those bonuses passively, and have more points to spend on fun and interesting talents. Talents will become much more personal, much less cookie-cutter, and overall more enjoyable.
By way of example, with Assassination Rogues increased Poison Damage will be part of the Assassination tree Mastery. On the other hand, Subtlety Rogues will have a Mastery bonus that reduces the Energy cost of abilities. Beast Mastery Hunters will find that their Mastery increases their pet's damage, while Marksman Hunters will find their bonuses come in the form of faster Focus regeneration.
By spending points in a talent tree you will get three kinds of passive bonuses. The first is one that makes you do your job better, so if it is a healing tree you'll get a passive bonus that increases your healing done, or if it's a tanking tree you'll likely get one that will increase your health or armor, or both. The second passive stat will be stat-related, like Haste or Critical Strike Rating (it will always be applicable to your class/spec). The third bonus will always be something purely spec related, such as the previously mentioned poison damage for Assassination Rogues.
Besides being just a system, Mastery is also going to be a stat found on gear. Mastery points on gear will apply directly to this third, spec specific bonus.